1. Field of the Invention
The invention relates to the field of marine propulsion systems, and more particularly, to hydraulically powered marine outdrives.
2. Description of the Related Art
Marine propulsion systems can be classified into three broad categories: inboard, outboard, and inboard/outboard. Inboard systems typically have an inboard engine that drives a propeller on a fixed propeller shaft extending through the hull or transom with steering provided by a separate rudder. Outboard systems typically have the entire engine, drive train and propeller in a single unit mounted to the transom with steering provided by rotating the entire unit. Inboard/outboard systems typically have an inboard engine with an outboard drive system (usually a propeller) mounted to the transom, with steering provided by rotating the outboard drive. Inboard/outboard systems offer more mobility than inboard systems, and greater horsepower than purely outboard units. The term “outboard drive” is often shortened to “outdrive” and refers to the fact that the entire drive unit apart from the engine and transmission are located overboard, normally on the transom of the boat. This feature is critical to the vessel's trim, tilt and steering operations. With this type of system propulsion is achieved when rotation is transmitted from an inboard mounted engine through some form of drive train to a propeller located below the water line. Instead of a rudder setup, steering is executed by changing the angle of the entire unit in a plane parallel to the water surface. By varying this angle, propeller thrust is redirected and the vessel's course altered. The ability to direct propeller thrust makes the vessel responsive and extremely maneuverable, a feature that appeals to both commercial and pleasure boat owners.
In some known inboard/outboard systems, rotation from the inboard engine is reduced by a transmission and then directly coupled to the outdrive by a universal joint. Power is then transmitted through an arrangement of clutches, bevel gears and shafts to the propeller located below the water surface. Such fixed gear ratio arrangements tend not to use fuel to the utmost efficiency. For example, accelerating a boat from a standstill requires more horsepower than any other time during operation, and this occurs when the engine is running at low rpm and producing very little horsepower. At that time engines are over fueled in order to create more horsepower. However most of this excess fuel that is delivered to the engine is exhausted and not used. Also, particular engines, and particularly diesel engines, have a peak performance within a narrow rpm range, so in fixed ratio systems, the engine will be operating efficiently in a limited number of boat speeds and so most often will be operating with reduced fuel efficiency, causing increased costs and pollution. As well, the universal joint which must penetrate the transom of the boat is both a weak link in the drive train, as well as a difficult area to seal.
Various designs have been proposed wherein the inboard engine is used to drive a hydraulic pump, and the hydraulic pump provides hydraulic fluid under pressure to an outboard reversible hydraulic motor which drives the propeller shaft, eliminating the need for a mechanical linkage through or over the transom. See, for example, U.S. Pat. No. 3,139,062 to Keefe, U.S. Pat. Nos. 3,587,511 and 3,847,107 to Buddrus, and U.S. Pat. No. 3,599,595 to James. In these disclosures, a hydraulic motor is mounted on the propeller shaft, below the water line. But such designs result in large drag due to the volume of the housing which is below the water line. To reduce drag requires reducing the cross-sectional profile of the housing, which necessarily reduces and limits the power output of the motor.
In U.S. Pat. No. 2,486,049 to Miller and U.S. Pat. No. 3,673,978 to Jeffrey et al. the hydraulic motor is mounted above the water line and connects to the propeller shaft through bevel gears. In U.S. Pat. No. 5,813,887 to Mark, the hydraulic motor above the water line connects to the propeller shaft by a chain drive. But all such mechanical linkages have inherent disadvantages found in vibrations and noise, limitations on turning angles, maintenance and repair, added lubrication requirements, and cost.
There is a need for a lower cost, lower maintenance, simpler, high performance hydraulic marine outdrive.